In high-frequency/high-capacity optical fiber communication systems, optical modulators embedded with waveguide-type optical modulation elements are frequently used. Among these, optical modulation elements in which LiNbO3 (hereinafter, also referred to as “LN”) having an electro-optic effect is used for substrates cause only a small optical loss and are capable of realizing broad optical modulation characteristics and are thus widely used for high-frequency/high-capacity optical fiber communication systems.
The optical modulation element using LN is provided with a Mach-Zehnder optical waveguide, an RF electrode configured to apply a radio-frequency signal that is a modulation signal to the optical waveguide, and a bias electrode configured to perform various kinds of adjustment so as to maintain modulation characteristics in the waveguide in a satisfactory manner. In addition, the electrodes provided in the optical modulation element are connected to an external electronic circuit via lead pins or a connector that is provided in a housing of the optical modulator in which the optical modulation element is accommodated.
On the other hand, with regard to a modulation form in the optical fiber communication systems, a transmission format such as QPSK (quadrature phase shift keying) and DP-QPSK (dual polarization-quadrature phase shift keying) which use multi-level modulation or in which multiplexing is introduced to the multi-level modulation becomes a main stream in pursuit of a recent increase in transmission capacity.
An optical modulator (QPSK optical modulator) that performs QPSK modulation or an optical modulator (DP-QPSK optical modulator) that performs DP-QPSK modulation includes a plurality of Mach-Zehnder optical waveguides having a box structure, a plurality of radio-frequency signal electrodes, and a plurality of bias electrodes (for example, refer to Patent Literature 1). Accordingly, the size of the housing of the optical modulator tends to increase, and thus there is a strong demand for, particularly, miniaturization.
As one countermeasure for coping with the demand for miniaturization, in the related art, there is suggested an optical modulator in which a push-on type coaxial connector provided in a housing of the optical modulator as an interface of the RF electrodes is substituted with lead pins similar to an interface of the bias electrodes, and a flexible printed circuit (FPC) configured to connect the lead pins to an external circuit substrate is provided.
For example, in the DP-QPSK optical modulator, an optical modulation element including four Mach-Zehnder optical waveguides each including an RF electrode is used. In this case, in a case where four push-on type coaxial connectors are provided in the housing of the optical modulator, it is difficult to avoid an increase in size of the housing, but when using the lead pins and the FPC instead of the coaxial connectors, it is possible to realize miniaturization.
In addition, the lead pins of the housing of the optical modulator, and a circuit substrate on which an electronic circuit configured to allow the optical modulator to perform a modulation operation is mounted are connected to each other through the FPC. Accordingly, it is not necessary to perform coaxial cable excess-length processing that is used in the related art, and thus it is possible to reduce a mounting space of the optical modulator in the optical transmission apparatus.
For example, the FPC that is used in the optical modulator is prepared by using a flexible polyimide-based material as a substrate (hereinafter, referred to as “FPC substrate”), and each of a plurality of through-holes provided in the vicinity of one end is electrically connected to each of the pads provided on the other end through a wiring pattern. In addition, a plurality of lead pins, which protrude from a bottom surface or a lateral surface of the housing of the optical modulator, are respectively inserted into the plurality of through-holes, and are fixed and electrically connected to the plurality of through-holes, for example, by using solder. The plurality of pads are fixed and connected to the circuit substrate, for example, by using solder. According to this, a radio-frequency signal, which is applied from the pads on the circuit substrate, is applied to a corresponding RF electrode of the optical modulation element through corresponding through-hole and lead pin, and thus high-frequency optical modulation is performed.
In the optical modulator using the FPC, as described above, it is possible to miniaturize the housing, and it is also possible to reduce a mounting space of the optical modulator on the circuit substrate, and thus it is possible to greatly contribute to miniaturization of the optical transmission apparatus.
FIG. 13A, FIG. 13B, and FIG. 13C are views illustrating a configuration of an optical modulator provided with the FPC in the related art. FIG. 13A is a top view of the optical modulator, FIG. 13B is a front view thereof, and FIG. 13C is a bottom view thereof. An optical modulator 1300 includes an optical modulation element 1302, a housing 1304 that accommodates the optical modulation element 1302, a flexible printed circuit (FPC) 1306, an optical fiber 1308 through which a light beam is input to the optical modulation element 1302, and an optical fiber 1310 that guides the light beam output from the optical modulation element 1302 to the outside of the housing 1304.
The housing 1304 is provided with four lead pins 1320, 1322, 1324, and 1326 which are respectively connected to four RF electrodes (not illustrated) of the optical modulation element 1302, and the lead pins 1320, 1322, 1324, and 1326 are inserted into the following through-holes 1420, 1422, 1424, and 1426 which are provided in the FPC 1306, and are fixed and electrically connected thereto, for example, by using solder.
FIG. 14 is a view illustrating a configuration of the FPC 1306. In the FPC 1306, four pads 1410, 1412, 1414, and 1416 are provided in parallel in the vicinity of one side 1400 on a lower side in the drawing along a direction of the one side 1400. In addition, four through-holes 1420, 1422, 1424, and 1426 are provided in parallel on another side 1402 side that is opposite to the side 1400, for example, along a direction of the side 1402. In addition, the four pads 1410, 1412, 1414, and 1416 are respectively electrically connected to the through-holes 1420, 1422, 1424, and 1426 by wiring patterns 1430, 1432, 1434, and 1436.
In addition, the four pads 1410, 1412, 1414, and 1416 are respectively fixed and electrically connected to pads of the external circuit substrate, for example, by using solder. According to this, RF electrodes of the optical modulation element 1302 provided in the optical modulator 1300 and an electronic circuit configured on the circuit substrate are electrically connected to each other. In this state, the optical modulator 1300 is mounted in the optical transmission apparatus. Furthermore, as illustrated in the drawing, typically, a shape of the FPC 1306 is a horizontally elongated rectangular shape having short sides in a signal transmission direction to make the wiring patterns very short and to suppress a microwave loss to a low value. In a case where the four pads 1410, 1412, 1414, and 1416 are provided as in an example illustrated in the drawing, the shape becomes a rectangular shape having dimensions of approximately 20 mm or less in a long-side direction and approximately 10 mm or less in a short-side direction.
FIG. 15A and FIG. 15B are views illustrating an example of a state in which the optical modulator 1300 is connected to the circuit substrate in which the electronic circuit is constructed. FIG. 15A is a view seen from an upper surface direction of the optical modulator 1300, and FIG. 15B is a cross-sectional arrow view taken along line BB in FIG. 15A. Furthermore, description of an internal configuration of the optical modulator 1300 in FIG. 15B is omitted.
For example, the optical modulator 1300 and a circuit substrate 1500 are fixed to a base 1502 inside the housing of the optical transmission apparatus. As illustrated in FIG. 15A, the FPC 1306 of the optical modulator 1300 extends from a connection portion with the lead pins 1320, 1322, 1324, and 1326 toward a left side in the drawing, and a left end thereof is bent in an oblique lower-left direction in the drawing in order for a left end to come into contact with the circuit substrate 1500 as illustrated in FIG. 15B. According to this, the pads 1410, 1412, 1414, and 1416 of the FPC 1306 are fixed and electrically connected to pads 1510, 1512, 1514, and 1516 on the circuit substrate 1500, for example, by using solder (FIG. 15A).
However, when the optical modulator with FPC is connected to a circuit substrate inside the optical transmission apparatus, and the optical transmission apparatus is transported and an operation thereof is initiated, peeling-off or cracks may occur in a solder-fixing portion or a solder-connecting portion between the lead pins 1320, 1322, 1324, and 1326 of the housing 1304 of the optical modulator and the through-holes 1420, 1422, 1424, and 1426 of the FPC 1306 due to vibration in an installation environment in the middle of the transportation or during the operation. As a result, radio-frequency characteristics may deteriorate or vary during reaching the lead pins 1320, 1322, 1324, and 1326 from the pads 1410, 1412, 1414, and 1416 of the FPC 1306, and thus a problem may be caused in optical modulation characteristics in the optical modulator 1300.
As described above, in the FPC 1306, the through-holes 1420, 1422, 1424, and 1426 are fixed and electrically connected to the lead pins 1320, 1322, 1324, and 1326 of the housing 1304 of the optical modulator during manufacturing of the optical modulator 1300, for example, by using solder. In addition, when using the optical modulator 1300 in the optical transmission apparatus, the pads 1410, 1412, 1414, and 1416 are fixed and electrically connected to the pads 1510, 1512, 1514, and 1516 on the circuit substrate 1500, for example, by using solder. At this time, as can be seen from FIG. 14, in the FPC 1306, a portion from the through-hole 1420 (that is, a solder-fixing portion or a solder-connecting portion with the lead pin 1320) to a side 1404 on a right side in the drawing, and a portion from the through-hole 1426 (a solder-fixing portion or a solder-connecting portion with the lead pin 1326) to a side 1406 on a left side in the drawing respectively constitute cantilever beams (respectively referred to as “first and second cantilever beams”) having a length L1, and vibrate in correspondence with vibration (environmental vibration) under an environment in which the optical modulator 1300 is installed. In addition, similarly, a portion from the through-holes 1420, 1422, 1424, and 1426 (that is, a fixing portion with the lead pins 1320, 1322, 1324, and 1326) to the pads 1410, 1412, 1414, and 1416 constitutes a double-supported beam having a length L2, and a portion from the through-holes 1420, 1422, 1424, and 1426 to the side 1402 on an upper side in the drawing constitutes a cantilever beam (referred to as “third cantilever beam”) having a length L3. The respective portions vibrate in correspondence with the environmental vibration (that is, in the rectangular FPC illustrated in FIG. 14, four vibration modes including vibration of the double-supported beam, and three kinds of vibration of the first to third cantilever beams).
As the environmental vibration, in addition to vibration during transportation or assembly of the optical modulator 1300 and the optical transmission apparatus that uses the optical modulator 1300, there may be many factors such as vibration of a heat radiation fan provided in the optical transmission apparatus, a cooling fan of an apparatus rack in which the optical transmission apparatus is accommodated, air-conditioning vibration of a room in which the optical modulation device is installed, various kinds of vibration transmitted to a building in which the optical modulation device is installed.
In addition, particularly, in a case where a frequency of the environmental vibration is close to a natural vibration frequency that is determined by substrate rigidity of the FPC 1306, the lengths L1, L2, and L3, and the like, the cantilever beam portion or the double-supported beam portion may vibrate with a great amplitude as in resonant vibration. As a result, peeling-off or cracks may occur at the solder-fixing portion or the solder-connecting portion between the through-holes 1420, 1422, 1424, and 1426 and the lead pins 1320, 1322 1324, and 1326 which functions as a fixed end of the beam portions.
Here, the natural frequency of the beam portions becomes lower as the lengths L1, L2, and L3 corresponding thereto are long, and enters a frequency range in which a power spectrum density in the environmental vibration is great. As a result, the natural frequency is susceptible to an influence of the environmental vibration. For example, in a case of the FPC 1306 having a configuration as illustrated in FIG. 14, the portion of the length L1 or L2 is more susceptible to the influence of the environmental vibration in comparison to the portion of the length L3. More specifically, for example, the portion of the length L1 in FIG. 14 vibrates as indicated by a bold-line arrow in FIG. 16A, and may cause peeling-off or cracks to occur, particularly, in the solder-fixing portion or the solder-connecting portion of the lead pins 1320 and 1326, and the portion of length L2 in FIG. 14 vibrates as indicated by a bold-line arrow in FIG. 16B and may cause peeling-off or cracks to occur in the solder-fixing portion or the solder-connecting portion of the lead pins 1320, 1322, 1324, and 1326.
Particularly, an inner diameter of the through-holes 1420, 1422, 1424, and 1426 is as small as several hundreds of μm similar to a diameter of the lead pins 1320, 1322, 1324, and 1326 when considering that a frequency of a radio-frequency signal applied to the optical modulation element 1302 may be up to approximately several tens of GHz, and the through-holes is more susceptible to the influence of the environmental vibration in comparison to a typical electronic apparatus that deals with lead pins of approximately 1 mm.